Injection molds are used to form a variety of objects from plastic materials. The molds usually consist of two halves, which are aligned by leader pins and bushings. A die fits within a cavity defined between the two halves of the mold. A representative mold is shown in U.S. Pat. No. 5,662,946, to Steven D. Pratt et al; the first or fixed mold half 102 and the movable mold half 109 are shown in FIG. 1 of the Pratt et al patent. Leader pins 114, that fit into corresponding bushings 116, are also depicted, and side-locks 128, 129 assist in the alignment, and registration, of the two mold halves during molding.
As the mold in Pratt et al closes, the initial alignment of the mold halves occurs as leader pins 114 enter bushings 115. However, as the mold is operated, at high speeds, during extended production runs, the alignment of the two halves of the mold degrades and becomes less accurate. Side locks 128, 129 start to wear and are unable to fully compensate for the reduced accuracy, and the mold loses its ability to produce products within acceptable tolerances.
In order to maintain more precise alignment between the mold halves of injection molds than can be provided by leader pins and cooperating bushings, improved mold locks were developed. Many of the mold locks relied upon the coaction of a T-shaped member with a U-shaped receptacle. The T-shaped member is secured to one mold half, while the U-shaped member is secured to the cooperating mold half.
The T-shaped member comprises a horizontally extending head with an extending tongue. The U-shaped member includes an upwardly opening pocket that receives the tongue. The sides of the tongue are parallel to one another, and the side walls of the upwardly opening pocket are parallel to one another. Minimal clearance, in the order of 0.0002 to 0.0004 inches, is provided between the sides of the tongue and the side walls of the receptacle. The mold locks are installed in the mold so that the adjacent, alignment surfaces of the tongue and receptacle are parallel to the direction of thermal expansion of the mold halves.
U.S. Pat. No. 5,762,977, granted Jun. 9, 1998 to Borislav Boskovic, discloses a mold lock employing a T-shaped member that cooperates with a receptacle with a U-shaped pocket to receive the tongue of the T-shaped member. Boskovic discloses lock 16 for guiding, and aligning, two mating parts 12, 14 of mold 10; the mold would usually include a core and a cavity to form a specific article. The lock comprises guide member 18 having a socket, or U-shaped pocket 22, and a locking member 20, which is shaped to be engaged in the pocket, as shown in FIG. 3.
Boskovic improved upon previous mold locks by providing a self purging feature for the purging of foreign material, from the socket or receptacle, when the locking member and socket are engaged. The self-purging feature assumed the form of sloped wall 30 in cavity 28, shown in FIGS. 4 and 5, and described in column 2, lines 51–60, of the Boskovic patent. Typically, four of the locks are utilized for each mold, with one lock secured to each side of the mold, as shown in FIG. 2.
Several embodiments of a mold lock are shown in U.S. Pat. No. 6,981,858, granted Jan. 3, 2006, to Klaus A. Wider. In the preferred embodiment of FIGS. 1–5, the mold interlocking device includes first portion 22 attached to mold half 24. The first portion includes head 50 with channel 66 that receives a cylindrical pin or roller 68, as shown in FIGS. 1 and 3; the head is received in pocket 30 of receptacle 26 that is secured to cooperating mold half 28, as shown in FIG. 4. Inserts may include rollers, plates, plates with lubricating plugs, and plates comprised of layers.
However, all mold locks are subject to wear, which reduces their accuracy over time. In order to minimize wear, mold locks are frequently made from high grade hardened alloy steel, and are coated with wear resistant surfaces, such as titanium nitride. Nonetheless, known mold locks suffer from the same problem, namely dry unlubricated metal surfaces rubbing against each other, resulting in wear and galling.
The rubbing interaction of dry, contacting surfaces may be compounded by inaccuracies in machining the U-shaped pocket in the receptacle of the mold lock, coupled with the nominal clearance, between wear surfaces, of 0.0002 to 0.0004 inches. Consequently, the fit between the T-shaped member and the U-shaped pocket in the receptacle may result in line-to-line contact of the metal surfaces.